Camera is a kind of video input device. The role of a lens in a camera is equivalent to that of human eyes, and the brightness and clearness of the images taken by the camera are dependent on the quality of the lens used. One type of lens manufacturing uses full wafers to manufacture lenses in parallel using wafer processing equipment and forms lens wafers. Since a wafer camera is comprised of one or more bonded lens wafers, the result is that the wafer camera is a 2D array of lenses. Typically the array of lenses is singulated, and then individually aligned and bonded to individual sensors to make the final camera. The singulated camera lens is typically bonded to the sensor using a die-level pick and place process using either active or passive alignment techniques.
As an example of a passive camera alignment technique, a wafer level camera can be formed according to the following steps: first, a sensor wafer and a lens wafer are singulated into individual sensor dies and individual lenses, respectively; then, a flange focal length (FFL) of each individual lens is measured; after that, the individual lens chips are picked and bonded to the sensor dies one by one according to the measured FFL. This method can prevent the camera lenses from being out of focus due to the variations in their FFLs, thus meeting the performance specifications of the products. Moreover, the camera lenses fabricated by using this method can also meet the requirements on accuracies in x-y-z directions. However, since this method needs to pick the lenses, focus each of them, and then bond the focused lenses to the sensor dies one by one, such a die-level pick and place process is extremely expensive.
An alternative method for focusing is to use an active focus method where the image from the camera is analyzed, and the best focus is found using a feedback from the image on the sensor. But this method is also very expensive.
To solve the above-mentioned problems, manufacturers have proposed a wafer-level bonding process to fabricate camera lenses where the lens and sensor wafers are bonded before singulation. Specifically, as shown in FIG. 1, first, a lens wafer and a sensor wafer are provided. The lens wafer includes lenses 10 arranged in an array. The sensor wafer includes sensors 20 arranged in an array, and each of the sensor 20 has a glass cover formed thereon (not shown in FIG. 1). Then, the lens wafer and the sensor wafer are aligned and bonded to form a bonded wafer. Finally, the bonded wafer is singulated to form individual camera lenses 30. This wafer-level bonding process is mainly used in the fabrication of low-end camera lenses with relatively loose FFL 40 tolerance, such as some low-end VGA (Video Graphics Array) products, so as to improve the productivity, where FFL refers to the distance from the flange reference plane to the focal point of a lens.
However, because of fabrication errors, every lens produced may have a variation in the FFL compared with that of the ideal lens. As shown in FIG. 1, reference number 40 indicates the ideal FFL. When light transfers along the optical path, the difference in FFL of an individual lens 10 may cause the lens to have a worse performance than others. In the case when a tight design tolerance is required, the variations in FFL will cause many camera lenses to be out of focus and fail to meet the performance specifications. Therefore, accurate focusing is very important for the production of high-end camera lenses, which are required to meet higher requirements of performance. However, problems still exist during the production of high-end camera lenses using the above-mentioned existing wafer-level bonding process. Manufacturers are still seeking for solutions to solve these problems.